1. Field of the Invention
The present invention relates to a drive apparatus for an ultrasonic motor.
2. Description of the Related Art
Recently, attention has been paid to an ultrasonic motor using the vibration of a vibrator such as a piezoelectric element, as a new motor replacing an electromagnetic motor. As compared with the conventional electromagnetic motor, the ultrasonic motor is advantageous, for example, in that it provides a low-velocity high thrust without any gear, in that it provides great holding force, in that it provides a long stroke, in that it provides a high resolution, in that it provides good silence, and in that it does not generate magnetic noise and is not affected by magnetic noise.
Furthermore, in the ultrasonic motor, an ultrasonic vibrator is pressed, via a driver which is a frictional member, against a driven member which is a relatively moving member, such that frictional force is generated between the driver and the driven member, and the driven member is driven by this frictional force.
On the other hand, in actually driving the ultrasonic motor, control techniques are required to appropriately damp the vibration in the ultrasonic motor at the time of stopping or reversing the ultrasonic motor. Under such circumstances, the following technique has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-369557:
In a control unit of a vibrating actuator disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-369557, when an alternating drive signal is applied to a piezoelectric element of the vibrating actuator to drive the vibrating actuator and the vibrating actuator is then stopped at a target position, a phase difference is detected between the applied voltage and the vibration of the vibrator of the vibrating actuator, and a vibration excitation signal is applied to the piezoelectric element for a predetermined time with a phase which is delayed, for example, about 90° with respect to the phase of the vibration of the vibrator, thereby finishing the stopping operation.
Thus, according to the control unit of the vibrating actuator disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-369557, vibration which counteracts free vibration in the vibrator is excited, and damping vibration is inhibited, thereby positioning the actuator at the target position.
However, the control unit of the vibrating actuator disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-369557 is characterized in that the phase difference between the drive signal and the vibration formed in the vibrator is changed (0 to 180 degrees) to counteract the vibration of the vibrator itself and decelerate or stop the actuator. Moreover, in order to achieve such control, a drive circuit is complicated due to the need for means for detecting the vibration of the vibrator, means for comparing the phases of the detected vibration and the drive signal, and means for delaying the phase of the drive signal on the basis of the comparison. This can be said to be a technically obstructive factor in reducing the size of the control unit.
Furthermore, in the control unit of the vibrating actuator disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-369557, the vibration which counteracts the free vibration in the vibrator is excited. That is, the vibrator is targeted for the inhibition of the vibration. Therefore, it is hard to say that the response and accuracy are higher than in the technique that inhibits the vibration of the vibrating system.